The inkjet recording method is abruptly overspread and still making a progress, because the material cost is low, high-speed recording can be performed, noises are less generated at the recording and color recording is easy. The inkjet recording method includes a continuous system of continuously flying a liquid droplet and an on-demand system of flying a liquid droplet according to image information signals, and the ejection system therefor includes a system of ejecting a liquid droplet by applying a pressure from a piezoelectric element, a system of ejecting a liquid droplet by generating a bubble in the ink under heat, a system using an ultrasonic wave, and a system of suctioning and ejecting a liquid droplet by an electrostatic force. As for the inkjet recording ink, an aqueous ink, an oil-based ink or a solid (fusion-type) ink is used.
The coloring matter used in such an inkjet recording ink is required to exhibit good solubility in a solvent such as water, enable high-density recording, provide a good color hue, be fast to light, heat and active gases in the environment (for example, an oxidative gas such as NOx and ozone, and SOx) and highly resistant against water and chemicals, ensure good fixing property and less blurring on an image-receiving material, give an ink having excellent storability, have high purity and no toxicity and be available at a low cost.
Above all, the coloring matter is strongly demanded to be fast to light, humidity and heat, particularly, at the printing on an image-receiving material having an ink-receiving layer containing a porous white inorganic pigment particle, to be fast to oxidative gases such as ozone in the environment and excellent in the water resistance.
On the other hand, in the case where a recorded image having a high optical density is formed, it is known that there arises a problem that as the image is dried, the coloring matter crystal deposits on the surface of the recording material and the recorded image reflects light to cause a so-called bronze phenomenon of emitting metallic gloss. This phenomenon tends to readily occur when the water solubility of the coloring matter is decreased so as to improve water resistance, light resistance or gas resistance or a hydrogen bonding group is introduced into the coloring matter structure. The generation of the bronze phenomenon not only incurs decrease in the optical density of the recorded image but also causes the recorded image to have a color hue greatly different from the desired color hue or lose the transparency. Therefore, it is one of important performances required of the inkjet ink to prevent the bronze phenomenon.
As regards the method for preventing the bronze phenomenon, there have been heretofore known a method of adding a specific nitrogen-containing compound (see, for example, JP-A-6-25575 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-6-228476, JP-A-6-248212, JP-A-7-228810, JP-A-7-268261, JP-A-9-12946 and JP-A-9-12949), a method of adding a specific heterocyclic compound (see, JP-A-8-259865), a method of adding a specific titanium compound (see, JP-A-8-337745), a method of adding an alkali metal ion (see, JP-A-7-26178), and the like. The bronze phenomenon may be prevented from occurring by adding such an additive, but the amount of the additive added may be increased because of its insufficient effect or the additive may decrease various performances of the ink and the quality of the recorded image, such as deterioration in the storage stability. For example, as described in JP-A-8-259865, when an alkanolamine is added to the ink, the bronze phenomenon can be prevented, but by the addition only in a small amount, the pH of the ink increases to 11 or more and the high pH ink not only adversely affects nozzles but also lacks in safety on erroneously contacting with a human body and moreover, decreases the printing grade or water resistance of the recorded image.
In this way, various effects can be obtained by using an additive, but it is difficult to use conventional additives while maintaining various performances. Particularly, in the case where the solubility and aggregating property of the coloring matter need to be taken account of, selection of the kind and amount of the additive is difficult Also, in using an ionic additive, the effect thereof on the counter ion must also be taken into consideration. Accordingly, a molecular design of the additive by an unconventional new idea and a substantial bronze phenomenon-inhibiting method using the additive are being demanded.
As regards the method for improving such bronze gloss of the recorded image, there are disclosed a method of using a specific additive in combination (see, for example, JP-A72005-105261), and a method for adding a specific additive in combination for improving the storage stability (particularly, light fastness and ozone gas fastness) of the recorded image (see, for example, JP-A-2006-89730, JP-A-2006-89731, JP-A-2006-89732 and JP-A-2006-89733). However, a method satisfying both the improvement of bronze phenomenon of the recorded image and the improvement of preservability of the recorded image at the same time in a high improvement level has not yet been found, and a method for more improvement is being demanded at present.